As shown in FIG. 7, a conventional valve is provided with split upper and lower valve bodies 37 and 38, in which a flow passage 43 is formed, a seat ring 39 fitted in the upper valve body 37 and the lower valve body 38, a stem 40 extending through the seat ring 39 and supported to be substantially perpendicular to the axis of the flow passage 43, and a disc-shaped valve element 41 rotatably supported in the flow passage 43 by the stem 40. The rotation of the stem 40 causes the valve element 41 to rotate to be pressed against or separated from the seat ring 39 so that the valve is opened or closed.
The valve is usually produced by individually manufacturing the upper valve body 37 and the lower valve body 38, the seat ring 39, the stem 40, and the valve element 41 by the respective manufacturing processes, thereafter assembling the upper valve body 37 and the lower valve body 38 with a bolt 42, and adjusting the assembly. However, the assembling process has the following problems.    1. When fitting the seat ring 39 onto the inner peripheral surface of the hollow cylindrical valve body, if the seat ring 39 has a high hardness, it is difficult to assemble them. If the seat ring 39 is made of a material such as fluorine resin, etc., which is difficult to deform, it is necessary to make and assemble split halves consisting of the upper valve body 37 and the lower valve body 38, thus leading to an increase in the number of the assembling processes.    2. When fitting the valve element 41 on the inner peripheral surface side of the seat ring 39, the valve element 41 compresses and deforms the seat ring 39 to provide a certain sealing performance, and therefore the diameter of the inner peripheral surface of the seat ring 39 is smaller than that of the outer peripheral surface of the valve element 41. Consequently, a large force is necessary to fit the valve element 41, which makes the assembling difficult. Particularly, when fitting the valve element 41 into the fluoroplastic seat ring 39, it is necessary to first assemble the valve element 41 into the fluoroplastic seat ring 39, and then assemble them together with the split upper and lower valve bodies 37 and 38, because it is otherwise difficult to assemble the valve element 41 into the fluoroplastic seat ring 39 already fitted to the hollow cylindrical valve body. This increases the number of assembling processes.
As a method of solving the above problems, FIG. 8 shows a valve having a valve body 44 which is symmetrically splittable along a split surface 49 perpendicular to a flange face 50 and passing through the axis of a stem 45 and which is provided with a large number of holes opening toward the split surface 49 for lightening, and fastening devices to join the split halves of the valve body 44 together at the split surface 49 (see Japanese unexamined Patent Publication No. 7-305779).
FIG. 9 shows another known valve having a valve body 51, which is symmetrically splittable along a split surface 56 parallel to a flange face 57 and passing through the axis of a stem 52 and which is provided with a large number of holes opening toward the split surface 56 for lightening, and fastening devices to join the split halves of the valve body 51 together at the split surface 56 (see Japanese unexamined Patent Publication No. 8-285094).
According to the publications, not only can the weight of the valve body 44 or 51 be reduced by forming the holes, but also secondary machining to form through holes in the inner periphery of the valve body 44 or 51 and in the stem 45 or 52 is not necessary. In addition a pair of valve body halves 44 or 51 can be molded by a single mold, because they are symmetrical with respect to the axis of the stem 45 or 52. As a result, facilitation of the assembly, cost reduction, and mass production can be realized.
However, as the fastening devices for the split halves of the valve body 44 or 51 is comprised of the clamp 46 or 53 consisting of split halves, the securing flange 47 or 54, and a bolt 48 or 55, it is inevitable that the number of components is increased and troublesome operations for fastening the valve body 44 or 51 are involved.
The present invention has been completed to eliminate the above-mentioned drawbacks in the prior art. An object of the present invention is to provide a valve manufacturing method in which a seat ring and a valve body can be easily assembled together without splitting the valve body, and to provide a valve.